Use a per-thread path for the export directory in import_column_family_test (#6962)

Summary:
This is required so that the test cases can safely be run in parallel.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6962

Test Plan: `make check`

Reviewed By: zhichao-cao

Differential Revision: D21980060

Pulled By: ltamasi

fbshipit-source-id: 616b7a0b686155d3874848b9098c67ad3f47efcc

db/import_column_family_test.cc

@@ -14,7 +14,7 @@ class ImportColumnFamilyTest : public DBTestBase {
   ImportColumnFamilyTest() : DBTestBase("/import_column_family_test") {
     sst_files_dir_ = dbname_ + "/sst_files/";
     DestroyAndRecreateExternalSSTFilesDir();
-    export_files_dir_ = test::TmpDir(env_) + "/export";
+    export_files_dir_ = test::PerThreadDBPath(env_, "export");
     import_cfh_ = nullptr;
     import_cfh2_ = nullptr;
     metadata_ptr_ = nullptr;